S. Arunachalam et al.
Such isotropic lines are usually observed either due to the
intermolecular spin exchange which can broaden the lines or
due to occupancy of the unpaired electrons in a degenerate
orbital. However, theEPRspectraofthecomplexes[RuCl(PPh3)2L1]
and [RuBr(PPh3)2L2] were recorded at liquid nitrogen temperature
(LNT),showingarhombicspectrum(Fig. 1)withthreedistinct<g>
values. The rhombicity of the spectra reflects the asymmetry of
electronicenvironmentsaroundrutheniuminthesecomplexes.[22]
The system chosen for the study was the coupling of phenyl
magnesium bromide with bromobenzene to give biphenyl
as the product. Bromobenzene was first converted into the
corresponding Grignard reagent. To the above reagent, the
complex was added and the mixture was heated under reflux
for 6 h. After workup, the mixture yielded biphenyl, which was
compared with an authentic sample. Only a very small amount of
biphenyl was isolated when the reaction was carried out without
the catalyst. It was observed that the new Ru(III) Schiff base
complexeswerebettercatalystthanthealreadyreportedbinuclear
Ru(III) complexes. The possible mechanism for the coupling of
PhMgBr with PhBr catalysed by Ru(III) complexes has already
reported.[27]
Electrochemistry
Electrochemical studies of new ruthenium(III) Schiff base com-
plexes complexes were carried out by cyclic voltammetry in
acetonitrile solution using a platinum disk working electrode
and Pt wire counter electrode. All the potentials were referenced
to an Ag–AgCl reference electrode. The cyclic voltammetric data
are given in the Table 4. All the complexes were electroactive
only with respect to the metal center. Among the 12 complexes,
[RuCl(PPh3)2L3] and [RuBr(PPh3)2L2] showed reduction potential
in the range ꢁ0.49 to ꢁ0.60 V with a peak-to-peak separation
value in the range 255–293 mV. The complexes [RuBr(PPh3)2L3]
and [RuCl(AsPh3)2L3] showed only oxidation potential in the range
0.87–0.89 V with a peak-to-peak separation of 18–38 mV. Com-
plexes showed redox couples with peak-to-peak separation values
(ꢀEp) ranging from 29 to 765 mV, revealing that this process is
at best quasi-reversible.[11,16] This is attributed to slow electron
transfer and adsorption of the complex onto the electrode surface.
Hence, from the electrochemical data it is clear that the
present ligand system is ideally suitable for stabilizing the
higher oxidation state of ruthenium ion and the electron
transfer reactions take place without gross changes in the
stereochemistry of the complexes.[23] For the oxidation of the
complexes [RuBr(PPh3)2L3] and [RuCl(AsPh3)2L3] and reduction of
the complexes [RuCl(PPh3)2L1] and [RuCl(AsPh3)2L1] the peak-
to-peak separation values (ꢀEp) fell in the range 18–85 mV,
suggesting a reversible one electron transfer process.[24]
Biocidal (Antibacterial and Antifungal) Activities
The in vitro cytotoxicity of the complexes was screened in
order to evaluate activity against Staphylococcus epidermidis,
Escherichia coli, Botrytis cinerea and Aspergillus niger at 0.25,
0.50 and 1% concentration and the results are shown in
Table 6. The results indicate that the ruthenium(III) Schiff base
complexes are more active when compared with standard
references streptomycin/co-trimazole against the same microbes
under identical experimental conditions. The variation in the
effectiveness of the different compounds against different
organisms depends on the impermeability of the microbial cells
or on the difference in the ribosome of the microbial cells.[11]
In general, the complexes were more active than the parent
ligands and ruthenium(III) starting complexes. The increase in
the antibacterial activity of the metal chelates with increase in
concentration is due to the effect of metal ion on normal cell
processes. Such an increase in activity of the metal chelates can
be explained on the basis of Overtone’s concept[11] and chelation
theory.[28,29] According to Overtone’s concept of cell permeability,
the lipid membrane that surrounds the cell favors the passage
of only rapidly soluble materials, due to which liposolubility is
an important factor that controls the antimicrobial activity. On
chelation, the polarity of the metal ion will be reduced to a greater
extentduetotheoverlapoftheligandorbitalandpartialsharingof
positivechargeofmetalionwithdonorgroups.Further,itincreases
the delocalization of π-electrons over the whole chelate ring
and enhances the liphophilicity enhances the penetration of the
complexes. This increased lipophilicity enhances the penetration
of the complexes into lipid membrane and blocks the metal
binding sites on enzymes of microorganisms. These complexes
also disturb the respiration process of the cell and thus blocks
the synthesis of proteins, which restricts the further growth of the
organism. Furthermore, the mode of action of the complexes may
involve formation of hydrogen bond through <C N group with
the active centers of cell constituents, resulting in interference
with the normal cell process.[30] The Schiff base ligands and
their ruthenium(III) complexes possess activities which are more
effectivethanthestandarddrugs.Thevariationintheeffectiveness
of the different compounds against different organisms depends
either on the impermeability of the cells of the microbes or on
differences in ribosomes of microbial cells.[30]
Catalytic Oxidation of Alcohols
Catalytic oxidation of primary and secondary alcohols by some
of the synthesized ruthenium(III) Schiff base complexes were
carried out in CH2Cl2 stirred under an oxygen atmosphere
at ambient temperature and the results are summarized in
Table 5. Benzaldehyde, cyclohexanone, propionaldehyde and
2-methylpropionaldehyde were formed from benzyl alcohol,
cyclohexanol, propan-1-ol and 2-methylpropanol, respectively.
After stirring for about 6 h, the carbonyl compounds were treated
with 2,4-dinitrophenylhydrazine, methanol and a few drops of
sulfuric acid. The yellow products obtained were quantified as
the 2,4-dinitrophenylhydrazone derivatives. Only a very small
amount of carbonyl compound was formed when the reaction
was carried out without the catalyst in the presence of oxygen
atmosphereatambienttemperature.Therelativelyhigherproduct
yield obtained for oxidation of benzyl alcohol compared with
cyclohexanol, propan-1-ol and 2-methylpropanol was due to the
fact that the α-CH unit of benzyl alcohol is more acidic than
cyclohexanol, propan-1-ol and 2-methylpropanol.[25,26] Catalytic
oxidation of primary and secondary alcohols showed moderate to
high conversion to their corresponding aldehydes or ketones.
Experimental
Reagents and Materials
Aryl–Aryl Coupling
The new ruthenium(III) complexes were also used as catalysts
for phenyl–phenyl coupling reactions, as shown in Table 5.
All the reagents used were analytical reagent grade. Solvents
were purchased from Merck (A.R. grade) and were purified and
c
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 491–498
Arunachalam
